It's your water pump/thermostat. Listen to what everyone is telling you. The water pump is moving more water when your revs are high. Hence the warm air when moving. If it was the control valve you would not have hot air at all. But its your money. Piss it away as you see fit.

I just don't see how it's the water pump or the thermostat. The water pump doesn't just fade out. The impeller always works. The thermostat doesn't control flow to the heater core at all. The heat works if the thermostat is closed or open. In my opinion it has to be the valve or something in that part diagram I posted.

OP, can you tell us your cars water temperatures while at idle warmed up and while driving (at say 60-70 mph) warmed up? This will tell us if your thermostat is broken.